The present invention relates to a polishing machine, more precisely relates to a polishing machine having: a plate holder rotatably provided to a base; a polishing plate, whose upper face is covered with a polishing cloth as a polishing face, mounted on the plate holder; and a press unit for pressing a surface of a work piece onto the polishing face so as to polish the surface of the work piece like a mirror face.
A conventional polishing machine for polishing surfaces of silicon wafers, etc. is shown in FIG. 9. In the polishing machine, a plurality of silicon wafers, etc. are simultaneously polished like mirror faces. An upper face of a polishing plate, which is rotated in a direction of an arrow A, is covered with a polishing cloth 108. A plurality of press units 110, which are provided above the polishing clothe 108, are respectively rotated in directions of arrows B. Each of the press units 110 presses the silicon wafer onto the polishing cloth 108 so as to polish the silicon wafer.
A partial sectional view of the polishing machine shown in FIG. 9 is shown in FIG. 10. As shown in FIG. 10, a plate holder 104 is rotatably provided in a base 100. The plate holder is rotated, together with a rotary shaft in the direction A, by a motor (not shown). A polishing plate 106 is mounted on the plate holder 104. As described above, the polishing cloth 108 is fixed on an upper face of the polishing plate so as to form the polishing face.
The press unit 110 is provided above the polishing plate 106 and covers an outer edge part thereof. The press unit 110 presses work pieces 105, e.g., the silicon wafers, onto the polishing face of the polishing plate 106. The work pieces 105 are adhered on a lower face of the press unit 110, which faces the polishing face of the polishing plate 106.
Lower surfaces of the work pieces 105 are polished by the polishing cloth 108 with slurry, which is supplied by a pipe 107.
Weight of the polishing plate 106 and pressing force of the press unit 110 are applied to an outer edge part of the plate holder 104, so the outer edge part of the plate holder 104 is downwardly deformed with respect to a center part thereof, which is connected to the rotary shaft 102. Thus, it is difficult to keep the polishing face of the polishing plate 106 flat. Thus, in the conventional polishing machine, a ball bearing 112 is provided between the outer edge part of the plate holder 104 and the base 100.
As shown in FIG. 10, by providing the ball bearing 112 between the outer edge part of the plate holder 104 and the base 100, the plate holder 104 is capable of rotating and keeping the polishing face of the polishing plate 106 flat even if the weight of the polishing plate 106 and the pressing force of the press unit 110 are applied to the outer edge part of the plate holder 104.
However, balls of the ball bearings 112 are not true spheres and have minute errors in size. The errors can be reduced but cannot be zero. Therefore, the plate holder 104, which is supported by the ball bearing 112, is moved on the balls of the bearing 112, which rotate with the movement of the plate holder 104, so that fine vibrations of the plate holder 104, which are caused by the minute errors, are occurred. The fine vibrations of the plate holder 104 makes fine projections like ripples on the polished surfaces of the work pieces 105, so that flatness of the work pieces cannot be improved.
Generally, the polishing step is a final step, so the work pieces, which have been polished by the polishing machine, are forwarded. However, in the case of the silicon wafers for semiconductor devices, the flatness of the polished surfaces must be higher, so the work pieces whose surfaces have fine ripple-shaped projections badly influence to final products.